Capacitive micromachined ultrasonic transducers have been emerging as an attractive alternative to piezoelectric transducers. They offer a larger set of parameters for optimization of transducer performance as well as ease of fabrication and electronic integration. The fabrication and operation of micromachined ultrasonic transducers have been described in many publications and patents. For example, U.S. Pat. Nos. 5,619,476; 5,870,351 and 5,894,452, incorporated herein by reference, describe the fabrication of capacitive-type ultrasonic transducers in which membranes are supported above a substrate by insulative supports such as silicon nitride, silicon oxide and polyamide. The supports engage the edges of each membrane. A voltage applied between the substrate and a conductive film on the surface of the membrane causes the membrane to vibrate and emit sound waves. The membranes can be sealed to provide operation of the transducers immersed in a liquid. The transducer may include a plurality of membranes of the same or different sizes and/or shapes. In operation, one or more multi-element transducers can be in arrays with the electrical excitation controlled to provide desired beam patterns.
The ultrasonic energy in the substrate is in the form of guided plate waves (Lamb waves) which radiate from the surface and propagate in the substrate resulting in cross-coupling between transducers of the array. These waves disturb the beam profile of the acoustic energy generated by the transducer especially at the critical radiation angle of the plate waves to the fluid medium. Ultrasonic waves propagating at the fluid-solid interface are also generated and propagated predominantly in the fluid along the transducer surface to disturb the beam pattern. These interface waves generated by an active transducer start an undesired vibration of the neighboring transducers after a time delay, effectively reducing the imaging bandwidth of the array in addition to the beam pattern disturbances.